This invention relates generally to signal detection, and in particular to a two-dimensional signal detector with dynamic compensation for broadband data network applications.
A conventional telephone transmission line is typically comprised of a pair of copper conductors that connect a telephone set to the nearest central office, digital loop carrier equipment, remote switching unit or any other equipment serving as the extension of the services provided by the central office. This pair of copper conductors, which is also referred to as a twisted pair, has its leads named as tip and ring. The tip and ring nomenclature is derived from the electrical contacts of an old-style telephone plug. A number of such twisted pairs are generally bundled together within the same cable binder group.
The demand for high bandwidth data transmission over existing telephone transmission lines has led to the development of digital subscriber line (DSL) technology. Several variations of DSL technology (referred to generically as xDSL or simply DSL) are evolving, such as SHDSL (symmetric high-bit-rate DSL), HDSL2 (second-generation high-bit-rate DSL), RADSL (rate adaptive DSL), VDSL (very high-bit-rate DSL), and ADSL (asymmetric DSL). In general, a digital subscriber line is comprised of two DSL modems coupled to one another by a twisted pair. The transmit (Tx) and receive (Rx) signals of DSL communications are carried by the twisted pair.
One challenge presented by DSL technology (as well as other broadband technology) is detecting signal arrival time accurately under various channel and receiving signal conditions. For example, the power of a received signal may vary over a range of 50 decibels (dB) due to factors such as the loop length of the transmission line. Conventional signal detection mechanisms detect received signals having an amplitude above a single pre-set threshold. There are a number of problems associated with such detection mechanisms. For example, the detection reliability is compromised when the signal being detected has a wide dynamic variation in power. In addition, the detection accuracy is compromised due to timing misalignment between when detection of the signal is indicated and when the signal actually arrives.
Assume, for example, that a broadband network has a five Megabaud data transmission rate with 100 symbols per data packet. Further assume that the duration of a data packet is 20 microseconds. If the signal detection indication is misaligned with the actual signal arrival by two microseconds, the receiver will miss ten data symbols. Such data loss may require retransmission of the associated data packet. Thus, misalignment between when detection of the signal is indicated and when the signal actually arrives can result in degraded system performance.
What is needed, therefore, is a signal detector that is reliable when detecting a signal that has a wide dynamic variation in signal power, and can dynamically correct for timing misalignment to improve detection time accuracy.
One embodiment of the present invention provides a method for detecting an input signal at a transceiver of a communication system. The method includes characterizing the actual arrival of the input signal by applying the input signal to a number of predetermined sequential threshold levels including a first threshold level. The method further includes noting the effective time at which each predetermined threshold level is crossed by the input signal. The method proceeds with indicating receipt of the input signal after a predetermined settling time, and identifying the last threshold level crossed. The last threshold level is associated with the actual arrival of the input signal. The method further includes compensating for timing misalignment between when the receipt of the input signal was indicated and when the input signal actually arrived.
Another embodiment of the present invention provides a signal detector for detecting an input signal. The signal detector includes a multilevel detector module having a number of predetermined sequential threshold levels including a first threshold level. The multilevel detector module is adapted to indicate each threshold level crossed by the input signal. The detector further includes a control logic module adapted to time-stamp the input signal at each threshold level reached, and to indicate receipt of the input signal upon expiration of a predetermined settling time. The detector also includes a memory for storing consecutive samples of the input signal. Each stored sample corresponds to a specific indexed location of the memory. The detector further includes a computation module adapted to calculate the index specifying the memory location storing the sample of the input signal that is associated with the actual arrival of the input signal.
These and other embodiments are described in more detail in the detailed description of the invention section. The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.